The exploitation and economically successful reovery of apatite in recent decades from the ultrabasic rock deposits of the Phalaborwa Igneous Complex in the Republic of South Africa constituted a major breakthrough in metallurgy because of the exceptionally low grade of the ore on the one hand and the vast reserves made available thereby on the other hand.
Initially the commercial activities were confined to an ore rock which became known as foskorite and in which the apatite percentage (though low by conventional standards) was higher than in the remainder of the complex. The key to the successful concentration and purification of apatite from foskorite (developed after prolonged experimentation with numerous ore-dressing methods) became a novel flotation process.
Some years later after prolonged and diverse experimentation, further flotation techniques had been invented and developed which also permitted the successful exploitation of the vast amounts of apatite in the even lower grade pyroxenite types of apatite ore of the complex. These processes have been in commercial use for some 10 years and have been so successful, both in respect of recoveries (up to 90%) and grade, that experts in the art were quite satisfied that flotation constituted the only effective answer to the problems of recovering apatite from this kind of rock.
In the circumstances, and particularly after experiments with sophisticated wet HIMS (HIMS=high intensity magnetic separation) techniques had yielded no useful results at all, it was firmly believed that the only prospects for further improvement lay in refinements of the flotation techniques. This view was adopted and accepted in spite of the drawbacks inherent in flotation, i.e.
(a) high energy requirements, PA1 (b) many moving parts in the equipment PA1 (c) high wear and tear, PA1 (d) costly chemicals, PA1 (e) large water requirements, PA1 (f) severe effluent disposal problems, PA1 (g) fines and slimes problems, including disposal thereof, PA1 (h) need for careful control and frequent adjustments, PA1 (i) sensitivity to variations in the ore and gangue minerals, PA1 (j) grinding the ore to a small particle size and attendant problems, PA1 (k) high capital and running costs of auxiliary equipment such as pumps, thickeners, filters, drying plant, dust control equipment etc. PA1 (a) a milling installation, followed by PA1 (b) dry classification equipment, followed by PA1 (c) dry HIMS apparatus.
Moreover, it was not possible to improve economically all quality aspects of the recovered apatite to satisfy all technical requirements including those of potential export markets. Particular problems were experienced with the MgO content of the recovered apatite, namely typically 2% MgO in the case of foskorite concentrates and typically 1.1% MgO in the more abundant pyroxenite concentrates. Even the latter MgO content is far above many acceptable limits.
For the manufacture of diammonium phosphate (a particularly popular fertiliser in overseas markets) a limit of 0.6% MgO is prescribed. For the manufacture of superphosphoric acid and technical high grade phosphate the upper limit is 0.3% MgO in the rock phosphate.
It is clear therefore that there has existed a real need for improvement in any one or more of the aforementioned respects.
The present invention is based on the quite surprising and unexpected result of careful experimentation that in certain circumstances to be more fully explained below, it is possible to attain very good recoveries of apatite from pyroxenite ore, often even better recoveries than by the best flotation techniques known to the applicant, and that moreover, the MgO-content is usually considerably lower than when the same ore is concentrated by flotation-often sufficiently low to satisfy the most stringent purity requirements by world standards.
These results are particularly surprising in the light of previous complete failures to attain useful results by wet HIMS methods, even though these experiments had been carried out by experts in those methods and with sophisticated equipment. Normally the results of wet HIMS are a reliable indication of results to be expected with dry HIMS techniques.
The applicant is also aware that dry HIMS has previously been used in relation to phosphate-containing ores, for example as disclosed in U.S. Pat. No. 3,022,956 granted on Feb. 27, 1962. However, such prior use of dry HIMS has been confined to the context of separating highly ferromagnetic material, for example, magnetic and similar iron-ore minerals, from apatite.
The pyroxenite ore of the Phalaborwa Igneous Complex comprising apatite, phlogopite and/or vermiculite and diopside is naturally an entirely different raw material when compared to magnetite-phosphate type ores, and hence presents an entirely different and new metallurgical problem. The crux of this difference is to be found in the absence or virtual absence of ferromagnetic minerals in these pyroxenite ores, with the result that the mineral components to be separated are either non-attracted or only weakly attracted. Magnetic separation would therefore not appear to provide a solution to the separation of the various mineral components present in these pyroxenite type ores.